1. Field of the Invention
The present invention relates to a method of and apparatus for vapor drying an object. More particularly, the present invention is concerned with a method and apparatus for drying objects such as semiconductor wafers and other semiconductor devices with the vapor of a drying medium such as alcohol or freon, i.e., a fluorocarbon.
2. Description of the Related Art
Referring to FIG. 3, a typical known apparatus for vapor drying an object has a substantially cylindrical vessel 1 which is closed at its bottom and open at its top. The vessel 1 holds a suitable amount of isopropyl alcohol 2. A heater 3 for heating the isopropyl alcohol 2 is placed under the bottom of the vessel 1. A waste liquid pan 4 is provided in the vessel 1. A cooling pipe 5 in the form of a coil is placed in the vessel 1 along the inner peripheral surface of the vessel 1 at an upper portion thereof. A refrigerant circulating means (not shown) is connected to the cooling pipe 5 to circulate a refrigerant through the cooling pipe 5.
In operation, the isopropyl alcohol 2 in the vessel 1 is boiled and evaporated by the heat from the heater 3. Meanwhile, a cold atmosphere is formed in the upper portion of the vessel 1 as a result of the recirculation of the refrigerant through the cooling pipe 5 by the refrigerant circulating means. In consequence, the vapor of the isopropyl alcohol rises to the upper portion of the vessel 1 where it is then cooled and condensed into a liquid phase and falls back into the liquid portion of the isopropyl alcohol at the bottom of the vessel 1. The isopropyl alcohol vapor is therefore prevented from escaping from the vessel 1 and fills up the vacant spaces in the vessel 1. In this state, the isopropyl alcohol vapor is held at a temperature substantially equal to the boiling temperature of isopropyl alcohol which is about 83.degree. C.
An object to be dried is set in the vessel 1. The object is, for example, a semiconductor wafer 6 that has been washed with water and still has moisture remaining on its surface. The portion of the alcohol vapor that contacts the surface of the semiconductor wafer 6 is cooled and liquefies to mix with the moisture on the surface of the semiconductor wafer 6. These droplets progressively increase in size due to the continuous condensation of the isopropyl alcohol and flow down along the surface of the semiconductor wafer 6. The droplets then drip into the waste liquid pan 4 and are discharged to the outside of the vessel 1 through a drain pipe 7.
Thus, the moisture on the surface of the semiconductor wafer 6 is replaced by the isopropyl alcohol.
Referring now to FIG. 4, the temperature T.sub.1 of the surface of the wafer 6 progressively increases after the moment t.sub.0 at which the wafer 6 is set in the vassel 1. On the other hand, the temperature T.sub.2 of the isopropyl alcohol 2 in the vassel 1 temporarily decreases due to the wafer 6 and then rises again to near the boiling point of 83.degree. C. When the temperature T.sub.1 of the surface of the wafer 6 has reached almost the temperature T.sub.2 of the isopropyl alcohol, which is about 83.degree. C., the liquid phase of the isopropyl alcohol, which has replaced the remaining moisture on the surface of the wafer 6, evaporates, whereby the drying of the wafer 6 is completed.
This known drying method has the following problem.
In this method, the drying of the object relies upon the replacement of the moisture remaining on the object with the drying medium, i.e., isopropyl alcohol, which liquefies upon contact with the relatively cool surface of the object. This replacement is effectively achieved only when the temperature of the surface of the object is below the temperature of the alcohol vapor. Thus, the time available for the replacement is limited and there is a risk that the water content on the object surface cannot be fully replaced by the alcohol with the result that the object is dried imperfectly.
This problem is serious particularly when the object to be dried has high thermal conductivity. In such a case the temperature of the surface of the object rapidly rises to that of the vapor temperature. Thus, the described known drying method does not always satisfactorily dry the object particularly when the object has high thermal conductivity.